MPLS over GRE
Another alternative to interconnect enterprise MPLS VPN networks over a WAN IP service is to create a mesh of GRE tunnels over the WAN IP cloud and enable label switching on the GRE interfaces. This setup achieves two benefits:
As shown in Figure 7-12, in this scenario, the WAN edge router plays the role of an E-P device.
Figure 7-12. MPLS over GRE
A p2p GRE tunnel is set up between each edge router pair if a full mesh is desired. From a
After the GRE tunnels have been established, the configuration of this solution exactly matches that used to deploy MPLS over Layer 2 circuits. The only difference is that instead of enabling label switching on the WAN interfaces, it is enabled on the tunnel interfaces. Therefore, the command-line interface (CLI) should look like this:
interface Tunnel0 description GRE tunnel to E-P2 bandwidth 1000 ip address 172.16.100.2 255.255.255.0 ip mtu 1400 tag-switching ip ip ospf network broadcast ip ospf priority 0 tunnel source Loopback0 tunnel destination 10.126.100.254
After the route/label distribution has been completed in the control plane, the enterprise edge device acts like a label switching router (LSR/P) where it treats the GRE interfaces as normal access interfaces. Figure 7-13 shows end-to-end packet flow between campus networks/MANs.
Figure 7-13. MPLS over GRE Packet Flow
The figure shows how the MPLS label (LDP2) is
The traffic inside the service provider cloud may or may not be MPLS switched, thus the service provider LDP and service provider VPN labels are present only if the service is an MPLS VPN. However, this should be of no concern to the enterprise and is mentioned here only for completeness.
Figure 7-14 shows an interesting variation of the use of VPNs over GRE tunnels. In this scenario, the WAN edge devices are also acting as E-PE devices. Note that this is equivalent to having the PE routers connected back to back. Because of PHP, no LDP label or even an LSP is
Figure 7-14. RFC 2547 VPNs over GRE Packet Flow
Clearly, this network is not using label switching, nor does it need label switching functionality. However, support for RFC 2547 VPNs is still required. Therefore, this type of overlay model is usually referred to as
2547 over X
. In this particular scenario, it would be 2547 over GRE. Note that this
At of the time this writing, support for 2547 over X is more common than support for MPLS over X.
2547 over GRE has been superseded by 2547 over mGRE, which incorporates all the flexibility of dynamic multipoint tunnel interfaces. The technical details and configuration of this approach are identical to those of the 2547 over L2TPv3 implementation. The sole difference is the encapsulation being used; all other architectural and configuration details are comparable. Therefore, we
Benefits and Drawbacks
Deploying MPLS (or RFC 2547) over a mesh of GRE tunnels allows the enterprise to extend their MPLS network over almost any IP network. As always, there are both challenges and benefits with this type of WAN extension.
The benefits of deploying MPLS over GRE tunnels include the following:
The drawbacks include the following:
The extension of MPLS VPNs over GRE tunnels is useful in scenarios that require the aggregation of a limited number of sites in a hub-and-spoke logical topology. Any-to-any connectivity for many sites is better addressed by dynamic mechanisms, such as CsC or RFC 2547 over DMVPN. DMVPN-based mechanisms are discussed in some of the following sections.